The present invention relates to a method of manufacturing a semiconductor device. More particularly this invention relates to a method of manufacturing a semiconductor device such as IC (integrated circuit) chips used for IC cards.
It has been expected in the future that there will be a wide distribution of IC cards such as railway ticket cards and electronic money cards in the field of electronic commerce. The IC cards are mounted with IC chips that store various kinds of information. The IC chips are generally constructed of nonvolatile memories, such as, for example, ferroelectric memories (FeRAM).
Information about fare and a section of a railway a person travels, etc. are stored in the IC chips used as the railway ticket card. Information about a password, amount, a bank account number, a credit number, etc. is stored in IC chips used as the electronic money card. It is important that these information must not be made known to other persons, and that the information regarding money can not be rewritten by a third party.
In general, a plurality of square chip areas, each chip area having a size of the order of a few millimeters on one side, are formed on a semiconductor wafer. The IC chips are then manufactured by cutting and separating these chip areas. FIG. 1 is a top plan view schematically illustrating an enlarged main portion of a prior-art semiconductor wafer. A plurality of chip areas 11 are formed on a semiconductor wafer 1.
Although not shown, circuit elements like transistors are formed in each chip area 11. A plurality of pads 12 are also formed within each chip area 11. The pads 12 are electrically connected to various circuit elements with wires 13. Four pads 12 are illustrated in this figure. However, in general, several to a hundred and dozens of pads 12 are present in one wafer.
A scribe area 14 is formed between the chip areas 11. The wafer 1 is cut along the scribe areas 14 using a diamond cutter or using laser beams at the time of dicing. Individual IC chips are obtained by such cutting.
In order to use these IC chips in the IC cards, it is necessary to write various information in a memory area within each chip before dicing, that is, when the IC chips are in the form of a wafer. As this information is extremely secret, it is necessary to provide various measures to prevent the information from being leaked to other persons or from being corrupted. Therefore, the technique of encryption or the like is utilized so as to prevent the written information from being easily stolen.
When the conventional IC chips having a structure as shown in FIG. 1 are to be used directly for IC cards, various pieces of secret information are written into the IC chips through the pads 12 before dicing. Accordingly, when IC cards mounted with the IC chips are handed over to users after dicing, there is a risk that the information written on the IC cards is read out through the pads 12 that remain in the chip areas 11. Further, if a person succeeds in decoding the read out information, then he can easily corrupt the written information through the remaining pads 12.
It is an object of the present invention to provide a method of manufacturing a semiconductor device capable of preventing highly secret information written in the semiconductor device before dicing from being read out or corrupted after dicing.
In order to achieve the above object, according to the present invention, on a semiconductor wafer, there are formed chip areas for storing memory areas, scribe areas for disconnecting the semiconductor wafer, pads for being supplied with electric signals from the outside for writing data into the memory areas, and lead wires for electrically connecting the pads with the memory areas. In this case, the pads are formed within the scribe areas. After data has been written into the memory areas through the pads, the semiconductor wafer is cut along the scribe areas, thereby obtaining semiconductor chips (IC chips).
According to the present invention, prior to the cutting of the semiconductor wafer, data is written into the memory areas within the chip areas by supplying electric signals to the pads formed in the scribe areas. The pads are cut together with the scribe areas at the time of dicing. Therefore, after the semiconductor wafer has been cut and semiconductor chips are obtained, it is possible to prevent the written data from being read out from the memory areas of the semiconductor chips and to prevent corruption of the written data.
The pads and the lead wires may be formed on the same metal wired layer. Alternatively, the pads and the lead wires may be formed on different wired layers, and they are electrically connected together through contact sections. Alternatively, in the present invention, the lead wires may be formed on a plurality of divided wired layers, and the lead wires are electrically connected with the pads through contact sections.
Further, after the data has been written into the memory areas within the chip areas, and before carrying out dicing, the pads in the scribe areas may be removed based on a photolithographic technique and by etching.
Based on the above-described arrangement, it is possible to prevent a part of each pad from remaining in each semiconductor chip after dicing when the width of the cut area in which semiconductor chips are actually obtained by dicing is smaller than the width of each pad. Accordingly, after the semiconductor wafer has been cut and the semiconductor chips are obtained, it is possible to prevent more securely the written data from being read out from the memory areas of the semiconductor chips and to prevent corruption of the written data.
Further, after the data has been written into the memory areas within the chip areas, the lead wires within the chip areas may be cut before dicing.
With the above-described arrangement, even if a part of the pad remains in the semiconductor chip after dicing, it is possible to isolate this pad from the inside memory area. Accordingly, after the semiconductor wafer has been disconnected to obtain the semiconductor chips, it is possible to prevent more securely the written data from being read out from the memory areas of the semiconductor chips and to prevent corruption of the written data.
As a method of cutting each lead wire, it may be so arranged that a fuse window is provided on a part of the lead wire in advance, and laser beams are irradiated onto this fuse window.
As an alternative method of cutting each lead wire, it may be so arranged that a field-effect transistor is formed in the middle of the lead wire in advance, and a high voltage is applied to a gate of this transistor thereby destroying the transistor.
With the above-described arrangement, even if a part of the pad remains in the semiconductor chip after dicing, it is possible to isolate this pad from the inside memory area. Accordingly, it is possible to prevent more securely the written data from being read out from the memory areas of the semiconductor chips and to prevent corruption of the written data.
As an alternative method of cutting each lead wire, it may be so arranged that two field-effect transistors are formed in series connection in the middle of the lead wire in advance, and one transistor is destroyed in a state that the other transistor closer to the memory area within the chip area has been turned off.
With the above-described arrangement, it is possible to prevent more securely the written data from being read out from the semiconductor chip after dicing and to prevent corruption of the written data. Further, it is also possible to prevent the memory area from being affected by the breaking of the transistor.
In the present invention, each lead wire may be connected to a pad within a cut area in which semiconductor chips are actually obtained at the time of dicing, or each lead wire may be arranged beyond the cut area.
With the above-described arrangement, the lead wire is cut in the middle. Therefore, even if a part of the pad remains in the semiconductor chip after dicing, it is possible to isolate this pad from the inside memory area. Accordingly, it is possible to prevent more securely the written data from being read out from the memory areas of the semiconductor chips and to prevent corruption of the written data.
Further, in the present invention, pads within each chip area may be formed within a scribe area sandwiched by two adjacent chip areas such that the pads extend beyond a cut area in which semiconductor chips are actually obtained by dicing.
Based on the above-described arrangement, even if a part of the pad remains in a semiconductor chip after dicing, this pad was originally formed in another semiconductor chip. Therefore, it is possible to isolate this pad from the inside memory area. Accordingly, it is possible to prevent more securely the written data from being read out from the memory areas of the semiconductor chips and to prevent corruption of the written data.
In the present invention, a plurality of pads may be formed in each chip area, and the lead wires of the respective pads are disposed in a mutually insulated state by crossing together in complex through a plurality of wired layers.
With the above-described arrangement, after the semiconductor chip has been cut from the semiconductor wafer, it is difficult to understand the wired routes of the lead wires when a semiconductor chip is observed from the above. Therefore, even if a part of the pad remains in a semiconductor chip after dicing, it is not possible to read the write data from the memory area of the semiconductor chip through any one of the pads. Accordingly, it is possible to prevent more securely the written data from being read out from the memory areas of the semiconductor chips and to prevent corruption of the written data.
Alternatively, in the present invention, the lead wires connected to the pads may be drawn out into the scribe area, and the lead wires may be separated from the pads in this scribe area. In this case, it may also be so structured that the lead wires within the scribe area are returned within the cut area in which semiconductor chips are actually obtained when the semiconductor wafer is cut so that the returned lead wires are connected to the pads formed within the original chip area. Further, alternatively, it may be so structured that the lead wires are connected to the pads formed in another chip area, for example, in an adjacent chip area, beyond the scribe area.
Based on the above-described arrangement, a part of the lead wires is disconnected together with the scribe area at the time of dicing, so that the lead wires are separated from the pads. In other words, it is possible to electrically isolate the lead wires from the pads. Accordingly, after the semiconductor wafer has been cut and individual semiconductor chips are obtained, it is possible to prevent the written data from being read out from the memory areas of the semiconductor chips and to prevent corruption of the written data.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.